Ask The Experts

It’s been a number of years since I’ve worked with silicone, so forgive my ignorance. Are there any major providers of systems—i.e., Stratasys, etc.—that manufacture rapid prototyping systems, other then subtractive, that are capable of producing/curing medical-grade silicones with thick or thin walls and in multidurometer configurations? The idea is to eliminate my molding process altogether. If such systems are not available, why? If they are, names would be greatly appreciated.

mpmn — Sun, 20 Dec 2229 22:03:45 +0000

Statasys and other companies, I believe, do offer rubber or “siliconelike” materials called TangoPlus, Tangogray, and TangoBlack. The resolution and accuracy available are generally good enough to help validate a design, but they are not a replacement for an actual silicone part. As far as why they have not made a rapid prototyping system for silicone, I am not entirely sure. I am guessing that it is due to the peanut butter consistency of silicone in its uncured state. I am sure that it would be very difficult to lay down a thin enough layer to accurately create a part, not to mention then having to heat it up to cure it.

I am currently working on a medical device in which a needle pierces through a silicone septum. I can’t use silicone oil. Is there any other medical-grade lubricant that can be used to aid the needle through the silicone?

mpmn — Thu, 17 Dec 2229 20:49:31 +0000

I am not aware of any other medical-grade lubricant. One additive we sometimes use in applications is Zonyl. Teflon can also be loaded into the silicone prior to molding (~15% by weight). This creates some added lubricity throughout the part.

Is there a good way to surface treat silicone in order to make the surface more dirt resistant so that it will not pick up fibers and other debris easily?

mpmn — Thu, 17 Dec 2229 20:45:23 +0000

A mat finish versus a polished/shiny finish will pick up slightly less FM or “dirt.” There are also surface treatments such as a parylene coating that create a lubricity on the surface and coat the silicone part. The use of ionizing fans to reduce static when handling the part will also reduce FM instances.

For a subcutaneous implantable device, if a small amount of leachable is detected, how do you define the leachable limit using animal experiments? This assumes that the device passed cytotoxicity testing already but still shows a small amount of leachable.

mpmn — Tue, 01 May 2012 17:55:33 +0000

If you detect a leachable using a chemistry technique, the next step is to use ISO 10993-17 to determine the toxicological impact of the leachable. Usually, a board-certified toxicologist is used to determine the safety of the leachable.

We are looking for a good adhesive to pot nitinol or titanium wire in a hole in a Noryl device. The hole is 0.156 in., while the wire is 0.125 in. We are looking for an adhesive that can fill the gaps well and can stand up to resterilization using various sterilization methods, including autoclave and EtO.

mpmn — Tue, 01 May 2012 17:45:05 +0000

If the number of sterilization cycles is limited to a few, you may be able to utilize a light-curable adhesive for your application. Dymax 1128A-M is a medical-grade adhesive that cures using UV and visible light and exhibits good adhesion to a variety of metals. Adhesion to Noryl would have to be tested.

If more than 10 cycles of autoclave sterilization are required, the best choice of adhesive family would be heat-curable or two-part epoxies. Masterbond and Epo-Tec offer a variety of medical-grade materials that are suitable for repeatable sterilizations.

It appears that there have been ‘legacy’ materials in widespread EU medical market use—for example, PVC-based tubing and gloves—for more than three decades. However, they are expected to fail the latest ISO 10993 biocompatibility testing, where PVC is suggested as a positive control. By what means are these materials allowed to be in use in the EU market in their current form and for new devices? Is there a ‘retrospective’ biocompatibility path for materials that have extensive use history with no known deleterious effects?

mpmn — Fri, 03 Feb 2012 21:30:06 +0000

Several materials are known to show signs of toxicity in certain biocompatibility tests but are still safe to use. The best examples are latex and nitrile. Latex is the positive control for the cytotoxicity test, and nitrile gloves fail the hemolysis test, yet these materials are still acceptable medical device materials. The first thing you must do is show that the known problem material is not ‘hiding’ other toxic components. This is done by removing the problem material from the test in which it is known to show a positive result. Then, you can evaluate the other materials and processes to determine that the results are being derived from that one problem material only. After that, you can use either an industry search or expert opinions to justify the acceptance of the failure.

We are ready to perform biocompatibility testing on a new catheter-securement device and need to know which tests must be performed and which can be skipped. This is a Class I device consisting of a skin adhesive bandage (3M) topped with a polypropylene living hinge clamp, which is lined with a PET absorbent pad. I probably should perform Kligman sensitization and intracutaneous injection, but do I need to perform cytotoxicity, pyrogenicity, or other type of testing?

mpmn — Fri, 03 Feb 2012 21:20:45 +0000

We would do agar overlay for cytotoxicity, skin sensitization, and primary skin testing, and that’s all.

We currently employ a solvent process using cyclohexanone to bond PVC tubing with an ABS molded hub. We are going to be switching from ABS to Pebax (thermoplastic elastomer). However, because we believe that there are issues with the cyclohexanone creating the bond with the Pebax that we desire, I’m looking for some information regarding our process. Is it appropriate to continue to solvent bond (maybe with a different solvent) or to switch to a new process (UV adhesive, for example)?

mpmn — Wed, 18 Jan 2012 21:10:05 +0000

Solvent bonding typically works with amorphous thermoplastics such as PVC, ABS, PC, PMMA, and PS. Pebax belongs to the family of thermoplastic elastomers and generally offers good resistance to solvents. Depending on its grade and softness, it may swell in certain solvents but will not behave like amorphous thermoplastics do. If you replace the ABS with Pebax, you need to switch to a new bonding process. UV-light curable adhesives are a good option, and I would recommend that you try Dymax medical-grade adhesives 204-CTH-F and 209-CTH. They both adhere well to PVC, Pebax, and several other commonly used plastics.

My company manufactures and distributes braces for orthopedic use, such as wrist braces, knee sleeves, and ankle supports. The products are mostly made out of fabrics. I am constantly being asked if biocompatibility testing is really required, considering the low risk of the devices themselves and the fact that the fabrics are used in the clothing industry. Are there any circumstances under which no testing would be performed at all? What tests would you recommend for these types of products?

mpmn — Wed, 05 Oct 2011 22:51:33 +0000

There are a few circumstances in which biocompatibility testing is likely not required:

If the same product is already on the market and has historical use data.
If the product, device, or materials do not involve patient contact.

Although we would agree that your orthopedic brace is a low-risk device, biocompatibility testing, or at least some type of material characterization demonstrating equivalence to a product already on the market, would be required. The type of testing required for this type of device would be:

Cytotoxicity testing; agar overlay test. This is a test for skin-contacting devices. If your device could potentially contact breached skin, we would suggest that you perform an MEM elution cytotoxicity test.
Primary skin irritation test for skin-contacting products.
Buehler sensitization test for skin-contacting products.

Do you know if Pebax bonds to polypropylene—for instance during an overmolding operation?

mpmn — Thu, 01 Sep 2011 22:56:37 +0000

The bond between these two polymers is typically mechanical, with engineered geometries to aid in the physical interlocking of the two materials. Premold surface preparations can be explored with your resin suppliers to possibly promote a more covalent surface between the two materials. However, this step will add to the assembly time and can be time-sensitive, based on the particular pretreatment method chosen.